release-config: disallow new duplicate flag declarations

Flags must only be declared in one place in the tree.  This change
prevents new ones from being added while we fix the bad flags already
present.

Bug: 352105274
Test: manual
Merged-In: I8c4bf2b2852685e84177500f28fe8908016082b9
Change-Id: I8c4bf2b2852685e84177500f28fe8908016082b9